Modem laser printing is generally accomplished by what is commonly known as the electrophotographic process. At the heart of the imaging process is an organic photoconductive (OPC) drum, and the drum typically includes an extruded aluminum cylinder coated with a non-toxic organic photoconductive material. There are six generalized stages to the electrophotography process: cleaning, conditioning, writing, developing, transferring and fixing.
Cleaning is the first stage in the imaging process. This stage prepares the OPC drum to receive a new latent image by applying a physical and electrical cleaning process. The physical cleaning of the OPC is typically accomplished by a drum-cleaning blade (or wiper blade) and a recovery blade. The wiper blade scrapes any excess toner from the drum and the recovery blade catches the toner and sweeps it into a waste hopper. In the electrical aspect of cleaning, the previous image on the drum must be cleared before a new one may be applied. The electrical cleaning of the OPC drum is performed by erasure lamps (usually corona wire technology) or a primary charge roller (PCR), which eliminate the previous latent image from the drum.
After the drum has been cleaned, it must be conditioned or charged to accept the image from the laser. A primary corotron (corona wire or PCR) applies a uniform negative charge (usually in the range of −600V to −720 V DC) to the surface of the drum.
Following the conditioning stage is the writing stage. According to this stage, a laser beam is used to discharge a conditioned charge to the drum surface. The conditioned charge creates a latent image on the drum. An aluminum base is connected to an electrical ground and the photoconductive material comprising the OPC becomes electrically conductive to ground when exposed to light (generally a laser). Therefore, the negative charges deposited onto the surface of the drum conduct to the aluminum base when exposed to light, creating the latent image. The latent image area will attract toner in a later stage.
The fourth stage is developing. At this stage, the latent image becomes a visible image. This stage generally requires four major components: toner, a developer roller assembly, a metering blade, and an AC/DC charge. Toner is attracted to the developer roller either by an internal magnet or by an electro-static charge. The roller carries the toner particles to a metering blade (a/k/a a doctor blade), where toner tumbles and creates a tribo-electric charge (friction) on the surface of the toner particles. The metering blade then provides for an evenly distributed amount of toner to pass to the OPC drum. Once the toner particle has passed beyond the doctor blade, it is ready to be presented to the OPC drum. The developer roller is then charged with an AC/DC charge from the High Voltage Power Supply. This charge allows the toner particles to “jump” from the developer roller and travel to the OPC drum where it is attracted to the latent image.
At this point, the toner image on the drum is transferred onto a sheet of paper. As the paper is passed under the OPC drum, it is passing over a transfer corotron assembly. The transfer corotron assembly places a positive charge on the back of the page, thus attracting the toner from the drum.
The sixth and final stage is fixing. Also known as fusing, this is the stage in which toner is permanently affixed to the paper. The fuser assembly typically includes a heated roller, a pressure roller, a heating element, a thermistor, a thermal fuse, and, sometimes, a cleaning pad. The heating element is typically placed inside the heated roller, which is usually constructed of aluminum with a Teflon coating. The roller is heated to approximately 355° F. (180° C.). The second roller is usually a rigid foamed silicon rubber. This second roller applies pressure to the heated roller. The paper passes between the two rollers and the heated roller melts the toner particles while the pressure roller presses the toner into the fiber weave of the paper.
As laser-printing technology has evolved, one of the primary focal points is the printing speed. There is a constant demand for higher print speeds. However, as print speed increases, the power required for the fixing or fusing stage becomes greater, as the toner requires a certain amount of energy to melt and fuse to the paper. Current fusing technology has thus come to a speed “ceiling,” where faster print speeds may require printers to have dedicated thirty-amp circuits to provide the necessary power to the heating element to keep up with the high print speeds. As speed demands continue to rise, the availability and feasibility of heating element power requirements to fuse the toner has become prohibitive. In addition, it has been a constant problem to apply an even heating distribution to the roller and the toner, leading to poorly fused images.
Further, during times when the printer is not in use, generally the user prefers that the printer, and especially the high energy absorbing heating element, revert to a low power or “sleep” mode. However, when the user does have a need to print either while the printer is in sleep mode or when the printer has been turned off completely, it generally takes significant time for the heating element to warm up before the printer is operational. In addition, the use of heating elements introduces other deleterious effects, usually necessitating the use of cooling apparatus to keep components that may be heat sensitive from overheating. Often the use of fans is necessary—adding again to the power requirements.
U.S. Pat. No. 5,212,526 ('526) discusses an alternative to the conventional fusing by introducing an apparatus for simultaneously transferring and fusing a toner solute in a UV-curable solution. That is, the transferring and fusing of the toner solute is accomplished at the same time. However, the apparatus taught by the '526 patent utilizes a large belt to advance the page, and this large belt is very specialized and expensive. The large belt taught by the '526 patent must be a photoreceptor and also be capable of transmitting UV rays—capabilities that at present are very expensive. Further, the '526 patent depends on intimate contact between the paper and the photoreceptor to simultaneously transfer and fuse the image to the page. In fact, the transfer of the image from the photoreceptor to the paper is dependant on greater adhesion of the toner image to the paper than to the photoreceptor. With the wide variety of papers, finishes, and toners presently used, it is a very difficult proposition, at best, to ensure acceptable print quality by creating greater adhesion to the paper than the photoreceptor. The problems with simultaneous transferring and fusing of the toner according to the '526 patent are further exacerbated by the need to partially pre-cure the toner solute with an air knife to attempt to facilitate adherence of the toner solute to the paper. The air knife adds additional expense and apparatus to a printer.
U.S. Pat. No. 5,232,812 ('812) discloses another alternate process for forming an image. However, the process disclosed by the '812 patent involves applying a separate layer of UV-curable liquid over the toner and does suspend the toner particles in the UV-curable liquid.